A typical communication receiver includes several circuit blocks to process a received signal. The receiver has an antenna which receives a received signal containing a carrier signal that is modulated with a data signal. The received signal is initially amplified by a variable-gain radio frequency (RF) amplifier. After amplification, a mixer mixes the received signal with a local oscillator signal to down-convert the received signal to an IF or baseband signal. The output of the mixer is an intermediate frequency (IF) signal that is modulated with the data signal. A variable-gain IF amplifier further amplifies the IF signal. A low-pass filter then filters the IF signal. The signal is then further amplified by a front-end amplifier before being passed to a demodulator. A conventional receiver also contains a conventional Automatic Gain Control (AGC) circuit that is configured to compensate for amplitude variations in the received signal over time.
The conventional AGC circuit in a receiver attempts to optimally load the front-end amplifier to mitigate changes in received radio frequency signal strength. When the received RF signal strength decreases to a point where the AGC circuit must amplify the received RF signal, the AGC circuit initially amplifies the received RF signal by increasing a gain of the RF amplifier. When the RF amplifier provides maximum amplification, yet additional amplification of the received RF signal is further required, the AGC circuit varies a gain of the IF amplifier to further amplify the received signal.
Unfortunately, when received RF signal strength is so high as to require minimal amplification, the conventional AGC circuit causes the RF amplifier to compress the signal that is input to the front-end amplifier. The compression occurs because the high RF signal strength drives the RF amplifier to the 1 dB compression point. As a result, the front-end amplifier saturates, further distorting the received signal. Thus, the conventional AGC circuit degrades receiver performance when the received RF signal strength is high.
Further, the AGC roles of the RF amplifier and the IF amplifier in the AGC circuit cannot simply be switched to resolve this problem because the IF amplifier has a relatively higher noise figure than the RF amplifier. Thus, if the IF amplifier provides initial AGC gain adjustment, the IF amplifier injects noise into the gain-adjusted RF signal even when the AGC circuit provides only slight amplification. This results in a lower signal-to-noise ratio (SNR) for the receiver when the AGC roles of the RF amplifier and the IF amplifier in the AGC circuit are switched.
Accordingly, what is needed is a circuit and method of automatic gain control that overcomes the shortcomings described above.